During duplex data transmission over a two-wire line, i.e. when there is simultaneous transmission in both directions (upstream and downstream), both the useful signal originating from the opposite station and echoes resulting from the original station's own transmitter occur at the respective receiver inputs. An echo is the undesired inputting of a transmission signal into the receiver of the same station, said transmission signal being superimposed on the desired transmission and thus causing interference with it. These echoes are primarily due to incorrectly adapted impedances of the hybrid connector. Whereas the interference echoes can be suppressed by selective filters in transmission methods with a separate frequency position, in transmission methods with identical frequency position special devices are necessary for echo compensation.
The compensation or canceling out of echoes can be carried out either in the time domain or in the frequency domain. When echoes are cancelled out in the time domain, the echo parameters are derived by sampling what is passing through the echo channel and the echo emulation involves a complex time domain convolution. During the canceling out of the echo in the frequency domain, the echo parameters are acquired by sampling the spectrum of the echo channel.
An echo emulation and an adaptive update process can then take place in the frequency domain, for which purpose the estimated value of the spectrum of the echo channel is used. Discrete multitone modulation (DMT) is a form of multicarrier, which is carried out with digital signal processing, an IFFT/FFT pair being used as modulation/demodulation vectors. FTT means “Fast Fourier Transform” and IFFT means “Inverse Fast Fourier Transform”. The known device can be applied to asymmetrical data transmission. By combining a cyclical convolution with a linear convolution the cyclical convolution can be carried out more quickly in the frequency domain with block multiplication, the short linear convolution being performed in the time domain. This is described, for example, in U.S. Pat No. 5,317,596. As the echoes which occur depend very greatly on the analogue switching and the parameters of the line used, it is generally necessary to set the echo compensator adaptively.
With frame-synchronous transmission in which the timing position of the transmission and reception FFT frames correspond precisely, the greater component of the echo can be compensated by using a frequency domain compensator, as is known, for example from U.S. Pat. No. 5,317,596. With this known compensator it is possible in principle to compensate only the periodic parts of the echoes at each frequency. In systems whose transient response extends over the length of the “cyclic prefix”, an a periodic echo component which cannot be compensated with the known device occurs additionally.
For this reason, a time domain echo compensator is additionally used, said compensator processing the sampled values of the transmission signal and subtracting the resulting sampled values from the sampled values of the reception signal. As the sampled values have to be processed with the sampling frequency of the transmission or reception signal, the computational capacity necessary for this is very large.
The invention is therefore based on the object of permitting the a periodic echo component to be compensated with a smaller computational capacity than in the prior art.
According to the invention, this object is achieved by means of the method according to claim 1 and by means of the arrangement according to claim 12. Preferred refinements of the invention are the subject-matter of the dependent claims.